A radiation measuring apparatus used to detect an electromagnetic radiation such as X-ray and gamma ray is sometimes configured to be able to specify an incidence direction of the electromagnetic radiation when an event has occurred in which the electromagnetic radiation is inputted to a detector of the radiation measuring apparatus. It becomes possible to specify a spatial distribution of radiation sources by specifying the incidence direction of the electromagnetic radiation. This is very useful in practical use.
One of the radiation measuring apparatuses that are configured to specify the incidence direction of the electromagnetic radiation is a Compton camera. The Compton camera is a radiation measuring apparatus that images a spatial distribution of radiation sources by using the Compton scattering. The Compton camera specifies the incidence direction of the electromagnetic radiation (e.g. X-ray, gamma ray) by using the Compton scattering, and generates the image showing the spatial distribution of radiation sources from the specified incidence directions.
FIG. 1 is a diagram showing the overview of a Compton scattering of gamma ray. When the gamma ray is inputted to substance, a photon of the incident gamma ray collides with an electron in the substance to be scattered. At this time, the electron with which the photon has collided spatters (the spattered electron is called “a recoil electron”) and a part of energy of the photon of the incident gamma ray is given to the electron. Thus, the energy of the scattered photon of gamma ray becomes lower than that of the incident gamma ray. Supposing that the energy of the photon of incident gamma ray is E0, the energy acquired by the recoil electron is E1, and the energy of the scattered photon of gamma ray is E2, the following equation (1) is met:E0=E1+E2  (1)
FIG. 2 is a diagram showing the principle of a Compton camera, especially, the principle of specifying the incidence direction of the electromagnetic radiation. Typically, the detecting section of Compton camera has a scatterer and an absorber. An event has occurred in which the photon of electromagnetic radiation inputted from a radiation source is scattered by the scatterer in the Compton scattering, and the photon to have been scattered in the Compton scattering is absorbed by the absorber in the photoelectric absorption, a scattering angle θ of the electromagnetic radiation (in other words, an angle between a line segment which links the radiation source and the position X1 and a line passing through positions X1 and X2) is given by the following equation (2):
[Number 1]
                              cos          ⁢                                          ⁢          θ                =                  1          -                                    m              e                        ⁢                                          c                2                            ⁡                              (                                                      1                                          E                      2                                                        -                                      1                                                                  E                        1                                            +                                              E                        2                                                                                            )                                                                        (        2        )            where me is a rest mass of an electron, and c is a speed of light. Also, E1 is the energy acquired by the recoil electron in the scatterer in the Compton scattering and E2 is the energy of the photon absorbed by the absorber.
The Compton camera estimates a spatial distribution of radiation sources based on data of the scattering angle θ of the electromagnetic radiation obtained in this way and images the spatial distribution of radiation sources. More specifically, a Compton cone (a cone surface configured as a set of points where radiation sources can be located) in each event is reconstructed from data of a position X1 where the Compton scattering has occurred in each event, a position X2 where the photoelectric absorption has occurred, and the scattering angle θ of the electromagnetic radiation in the Compton scattering, and an image corresponding to the superposition of Compton cones of the respective events is generated as a radiation source distribution image showing the spatial distribution of radiation sources.
As would be understood from the above, the scattering angle θ of the electromagnetic radiation can be calculated only from the position X1 where the Compton scattering has occurred in each event, the energy E1 acquired by the recoil electron in the Compton scattering, and the energy E2 of the photon absorbed by the absorber. The position of the radiation source is specified only as a point on a circle where the radiation source can exist. However, if a recoiling direction of the recoil electron (a direction to which the recoil electron recoils) can be specified, another circle (another cone surface) can be defined based on the recoiling direction of the recoil electron. Thus, theoretically, it is possible to specify the incidence direction of the electromagnetic radiation as a direction which passes intersecting points of two circles. However, actually, the incidence direction of the electromagnetic radiation is specified as an arc due to the fluctuation of energy of the recoil electron, as shown in FIG. 3. For example, such a technique is disclosed in the international publication WO 2007/145154 (Patent Literature 1). The Compton camera which confines the incidence direction of the electromagnetic radiation based on the recoiling direction of the recoil electron is sometimes called an electron tracking-type Compton camera.
As the techniques in relation to the present invention, the international publication WO 2011/001610 (Patent Literature 2) discloses a gamma ray direction detecting apparatus having a detector including a plurality of detection pixels which detect gamma ray.